Pathway/XM System Management Manual

Configuring Clients and Requesters
Compaq NonStop™ Pathway/XM System Management Manual—426761-001
5-19
Setting TMF Parameters
When you include a TIMEOUT clause in the SET SERVER statement, all SEND
operations to that server class are timed by the TCP. If the specified number of seconds
elapses after a SEND operation is initiated and before a reply is received, the TCP issues
a Guardian CANCEL procedure call against the outstanding I/O to the server. If the
SEND operation was performed while the requester program was in transaction mode,
the transaction is automatically aborted by the file system. In such a case, the requester
program discovers that the transaction was aborted when it subsequently attempts to
update the database (with another SEND statement) or issues an END-TRANSACTION
statement.
TCP Checkpointing Strategy
In a Pathway/XM environment with the TMF subsystem running, the checkpointing
strategy followed by the TCPs is affected by the settings you specify for the TMF
parameter in the SET SERVER statements you use to configure your server classes.
The TCPs use the following strategy for checkpointing:
•
At the BEGIN-TRANSACTION statement, a full copy of the task’s context is made
to a secondary area (slot 1) in the extended data segment, and a checkpoint to the
backup is performed.
•
At the END-TRANSACTION statement, a full-context checkpoint is performed.
•
At the SEND statement with the SET SERVER TMF parameter defined as OFF, a
checkpoint is performed before and after the SEND statement when the SCREEN
COBOL program is outside of transaction mode. (Any time a SEND operation is
performed outside of a transaction boundary and the server attempts to lock or
update a record in an audited file, the operation fails with a Guardian error 75.)
•
At the SEND statement with the SET SERVER TMF parameter defined as ON, no
checkpoints are performed, whether or not the SCREEN COBOL program is in
transaction mode. This means that SEND requests to TMF protected servers that
operate on audited files require fewer checkpoints than SEND requests to servers
that do not operate under TMF protection.
TCP checkpointing requirements can be significantly reduced if Pathway applications
running with the TMF subsystem have TMF protected servers read outside of
transaction mode before updating the database.
You can improve the performance of a Pathway application by taking advantage of the
TCP checkpointing strategy for TMF protected servers, as follows:
•
Do not use transaction mode for a server with read-only access to a database if the
requester displays the data before any attempt is made to change the data. In the
event of a failure, the read operations are retryable and fault-tolerant operation is
maintained.
•
Do not use transaction mode for a server that writes to an entry-sequenced logging
file in which duplicates are acceptable. In the event of a failure, the requester can
Caution. If a SEND request outside of transaction mode is sent to a TMF protected server that
operates on nonaudited files, data might be lost because the TMF subsystem is not invoked
and the TCP performs fewer checkpoints.